This invention is directed to a method for increasing retention and drainage in a papermaking furnish using structurally rigid nonionic and anionic polymers. The structurally rigid polymer may be used alone or in combination with one or more conventional coagulants, flocculants and/or microparticles.
In the manufacture of paper, a papermaking furnish is formed into a paper sheet. The papermaking furnish is an aqueous slurry of cellulosic fiber having a fiber content of about 4 weight percent (percent dry weight of solids in the furnish) or less, and generally around 1.5% or less, and often below 1.0% ahead of the paper machine, while the finished sheet typically has less than 6 weight percent water. Hence the dewatering and retention aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
Gravity dewatering is the preferred method of drainage because of its relatively low cost. After gravity drainage more expensive methods are used for dewatering, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like. In actual practice a combination of such methods is employed to dewater, or dry, the sheet to the desired water content. Since gravity drainage is both the first dewatering method employed and the least expensive, an improvement in the efficiency of this drainage process will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost is retention of furnish components on and within the fiber mat. The papermaking furnish represents a system containing significant amounts of small particles stabilized by colloidal forces. The papermaking furnish generally contains, in addition to cellulosic fibers, particles ranging in size from about 5 to about 1000 nm consisting of, for example, cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the mat formed by the cellulosic fibers on the papermachine.
Greater retention of fines, fillers, and other components of the furnish permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes more important because the fines content of such lower quality pulps is generally greater. Greater retention also decreases the amount of such substances lost to the whitewater and hence reduces the amount of material costs, the cost of waste disposal and the adverse environmental effects therefrom. It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation may be determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally declines.
Various chemical additives have been utilized in an attempt to increase the rate at which water drains from the formed sheet, and to increase the amount of fines and filler retained in the sheet. The use of high molecular weight water-soluble polymers is a significant improvement in the manufacture of paper. These high molecular weight polymers act as flocculants, forming large flocs which deposit on the sheet. They also aid in the dewatering of the sheet.
The high molecular weight water-soluble polymer is typically added after a high shear point in the stock flow system leading up to the headbox of the paper machine. This is necessary since flocs are formed primarily by a bridging mechanism and their breakdown is a largely irreversible process. For this reason, most of the retention and drainage performance of a flocculant is lost by feeding it before a high shear point. To their detriment, feeding high molecular weight polymers after the high shear point often leads to formation problems. The feed requirements of the high molecular weight polymers and copolymers which provide improved retention often lead to a compromise between retention and formation.
While successful, high molecular weight flocculent programs may be improved by the addition of so called inorganic xe2x80x9cmicroparticlesxe2x80x9d such as copolymers of acrylic acid and acrylamide; bentonite and other clays; dispersed silica based materials; colloidal borosilicate; and naphthalene sulfonate/formaldehyde condensate polymers. The microparticle may be used along with a flocculant as part of a single polymer/microparticle retention and drainage program or along with a coagulant and a flocculant as part of a dual polymer/microparticle retention and drainage program.
In a single polymer/microparticle retention and drainage aid program, a flocculant, typically a cationic polymer, is the only polymer material added along with the microparticle. In such a program, a high molecular weight linear cationic polymer is added to the aqueous cellulosic papermaking suspension before shear is applied to the suspension, followed by the addition of a microparticle such as copolymers of acrylic acid and acrylamide; bentonite and other clays; dispersed silica based materials; colloidal silica; or naphthalene sulfonate/formaldehyde condensate polymers after the shear application. Shearing is generally provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shear breaks down the large flocs formed by the high molecular weight polymer into microflocs. Further agglomeration then ensues with the addition of the microparticle.
Although, as described above, the microparticle is typically added to the furnish after the flocculant and after at least one shear zone, the microparticle effect can also be observed if the microparticle is added before the flocculant and the shear zone.
Another method of improving the flocculation of cellulosic fines, mineral fillers and other furnish components on the fiber mat using a microparticle is in combination with a dual polymer program which uses, in addition to the microparticle, a coagulant and flocculant system. In such a dual polymer/microparticle system one or more coagulants are first added, for instance a low molecular weight synthetic cationic polymer and/or cationic starch. The coagulant may also be an inorganic coagulant such as alum or polyaluminum chlorides. This addition can take place at one or several points within the furnish make up system, including but not limited to the thick stock, white water system, or thin stock of a machine. This coagulant generally reduces the negative surface charges present on the particles in the furnish, such as cellulosic fines and mineral fillers, and thereby promotes a degree of agglomeration of such particles. However, in the presence of other detrimental anionic species, the coagulant serves to neutralize the interfering species enabling aggregation with the subsequent addition of a flocculent. Such a flocculant generally is a high molecular weight synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into larger agglomerates. The presence of such large agglomerates in the furnish, as the fiber mat of the paper sheet is being formed, increases retention. The agglomerates are filtered out of the water onto the fiber web, whereas unagglomerated particles would, to a great extent, pass through such a paper web. In such a program the order of addition of the microparticle and flocculant can be reversed successfully.
However, there is continuing need to develop new methods of improving the retention and drainage performance of the papermaking furnish, thereby increasing the efficiency of pulp or paper manufacture.
Structurally rigid polymers have been used as substitutes for pulp in papermaking (U.S. Pat. No. 4,749,753; Japanese Patent Application 1987-29251), but not as process additives. We have discovered that adding structurally rigid polymers to papermaking furnishes results in a substantial improvement of the retention and/or drainage properties of the furnishes.
Accordingly, in its principal embodiment, this invention is directed to a method of increasing retention and drainage in a papermaking furnish comprising adding to the furnish an effective amount of a structurally rigid nonionic or anionic polymer.
xe2x80x9cStructurally rigid polymersxe2x80x9d means polymers having a structure where the rotational conformation (degrees of freedom) of the polymer is restricted compared with common flexible polymeric materials. Structural rigidity is imparted to the polymers by incorporating rigid components such as alkenyl, alkynyl, cyloalkyl, heterocyclyl, aryl and heteroaryl groups along the main chain of the polymer. The structurally rigid polymers may be composed entirely of rigid components, or the rigid components may be connected by flexible chains such as alkyl or ether groups, so long as introduction of the flexible groups does not substantially effect the overall rigidity of the polymer. Further, the structurally rigid polymers should be water-soluble or water-dispersible and be nonionic or anionic, preferably anionic. The structurally rigid polymers have a molecular weight of from about 2000 to about 2,000,000, preferably from about 50,000 to about 200,000.
xe2x80x9cAryldiaminexe2x80x9d means a heteroaryl or aryl group substituted by two amino (xe2x80x94NH2) groups. The amino groups are separated by at least one ring atom, preferably by at least two ring atoms. Representative aryldiamines include, but are not limited to 4,4xe2x80x2-diamino-2,2xe2x80x2-bipenyldisulfonic acid, 4,4xe2x80x2-diamino-3,3xe2x80x2-bipenyldisulfonic acid, 4,4xe2x80x2-diamino-2,2xe2x80x2-bipenyldisulfonic acid, 3,3-diamino-5,5xe2x80x2-biphenyldisulfonic acid, 4,4-diamino-5,5xe2x80x2-dimethyl-2,2xe2x80x2-biphenyldisulfonic acid, 4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid, 3,3xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid, 2,5-diaminobenzenesulfonic acid, 2,4-diaminobenzenesulfonic acid, 3,5-diaminobenzenesulfonic acid, 2,5-diaminobenzene-1,4-disulfonic acid, 3,7-diaminonaphthalene-1,5-disulfonic acid, 3,7-diaminonaphthalene-2,6-disulfonic acid, 5,8-diaminonaphthalene-2,3-disulfonic acid, 4,8-diaminonaphthalene-2,6-disulfonic acid, 4,8-diaminonaphthalene-1,5-disulfonic acid, 5-amino-2-[1-(4-amino-2-sulfophenyl)-isopropyl]benzenesulfonic acid, 5-amino-2-(4-amino-2-sulfophenoxy)benzenesulfonic acid, 5-amino-2-[2-(4-amino-2-sulfophenyl)ethynyl]benzenesulfonic acid, and the like. Preferred aryldiamines are 4,4-diamino-2,2xe2x80x2-bipenyldisulfonic acid and 4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid.
xe2x80x9cCyclic dicarboxylatexe2x80x9d means a cycloalkyl, heterocyclyl, heteroaryl or aryl group substituted by at least two activated carboxyl groups where two of activated carboxyl groups are separated by at least one ring atom, preferably by at least two ring atoms. Representative cyclic dicarboxylates include, but are not limited to benzene-1,4-dicarbonyl chloride, benzene, 1,3-dicarbonyl chloride, 4,4xe2x80x2-biphenyldicarbonyl chloride, 2,6-naphthalenedicarbonyl chloride, 2,7-naphthalenedicarbonyl chloride, 1,5-naphthalenedicarbonyl chloride, 1,4-naphthalenedicarbonyl chloride, 1,2,4,5-benzenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,1,3-trioxo-6-[(1,1,3-trioxo-7-sulfobenzo[3,4-c]1,2-oxathiolen-6-yl)sulfonyl]benzo[c]1,2-oxathiolene-7-sulfonic acid, and the like.
A preferred cyclic dicarboxylate is benzene-1,4-dicarbonyl chloride.
xe2x80x9cActivated carboxy groupxe2x80x9d means a carboxylic acid group that has been converted to a group that will readily react with an amino group to form an amide bond. Representative activated carboxy groups include acid halides, haloformates, activated esters and carbonates.
xe2x80x9cAnionic substituentxe2x80x9d means a substituent that is negatively charged somewhere in a pH range of from about 1 to about 11. Preferred anionic substituents include xe2x80x94SO3Hxe2x80x94, xe2x80x94CO2Hxe2x80x94, xe2x80x94OPO3H2. A more preferred anionic substituent is xe2x80x94SO3H.
xe2x80x9cAlkylxe2x80x9d means a monovalent group derived from a straight or branched chain saturated hydrocarbon by the removal of a single hydrogen atom. Representative alkyl groups include methyl, ethyl, n- and iso-propyl, and the like.
xe2x80x9cAlkoxyxe2x80x9d and xe2x80x9calkoxylxe2x80x9d mean an alkyl-Oxe2x80x94 group wherein alkyl is defined herein. Representative alkoxy groups include methoxyl, ethoxyl, propoxyl, butoxyl, and the like.
xe2x80x9cAlkylenexe2x80x9d means a divalent group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms. Representative alkylene groups include methylene, ethylene, propylene, and the like.
xe2x80x9cAlkenylenexe2x80x9d means a divalent group derived from a straight or branched chain hydrocarbon containing at least one carbon-carbon double bond. Representative alkenylene include xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CH2CHxe2x95x90CHxe2x80x94, xe2x80x94C(CH3)xe2x95x90CHxe2x80x94, xe2x80x94CH2CHxe2x95x90CHCH2xe2x80x94-, and the like.
xe2x80x9cAlkynylenexe2x80x9d means a divalent group derived by the removal of two hydrogen atoms from a straight or branched chain acyclic hydrocarbon group containing a carbon-carbon triple bond. Representative alkynylene include xe2x80x94CHxe2x89xa1CHxe2x80x94, xe2x80x94CHxe2x89xa1CHxe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x89xa1CHxe2x80x94CH(CH3)xe2x80x94, and the like.
xe2x80x9cArylxe2x80x9d means an aromatic monocyclic or multicyclic ring system of about 6 to about 20 carbon atoms, preferably of about 6 to about 10 carbon atoms. Aryl also includes ring systems where two aryl groups are connected through alkylene, alkenylene or alkynylene groups. The aryl is optionally substituted with one or more alkyl, alkoxy or haloalkyl groups. Representative aryl groups include phenyl, biphenyl, naphthyl, cis- and trans-stilbene, biphenylmethyl, diphenylacetylene, and the like. The aryl is preferably substituted with one or more anionic substituents as defined herein.
xe2x80x9cCycloalkylxe2x80x9d means a non-aromatic mono- or multicyclic ring system of about 5 to about 10 carbon atoms. Preferred ring sizes of rings of the ring system include about 5 to about 6 ring atoms. The cycloalkyl is optionally substituted with one or more substituents selected from alkyl, alkoxy and haloalkyl. Representative monocyclic cycloalkyl include cyclopentyl, cyclohexyl, cycloheptyl, and the like. Representative multicyclic cycloalkyl include 1 -decalin, norbornyl, adamant-(1- or 2-)yl, and the like.
xe2x80x9cHeteroarylxe2x80x9d means an aromatic monocyclic or multicyclic ring system of about 5 to about 10, preferably from about 5 to about 6 ring atoms, in which one or more of the atoms in the ring system is/are element(s) other than carbon, for example nitrogen, oxygen or sulfur. Heteroaryl also includes ring systems where two aryl groups are connected through alkylene, alkenylene or alknynylene groups. The heteroaryl is optionally substituted with one one or more substituents selected from alkyl, alkoxy and haloalkyl. Representative heteroaryl groups include pyridyl, 4,4-dipyridinyl, quinolyl, fliryl, benzofuryl, thienyl, thiazolyl, pyrimidyl, indolyl, and the like.
xe2x80x9cHeterocyclylxe2x80x9d means a non-aromatic saturated monocyclic or multicyclic ring system of from about 5 to about 10 ring atoms, in which one or more of the atoms in the ring system is/are element(s) other than carbon, for example nitrogen, oxygen or sulfur. Preferred ring sizes of rings of the ring system include about 5 to about 6 ring atoms. The heterocyclyl is optionally substituted by one or more alkyl, alkoxy or haloalkyl groups. Representative heterocyclyl rings include piperidyl, pyrrolidinyl, piperazinyl, morpholinyl, thiomorpholinyl, thiazolidinyl, 1,3-dioxolanyl, 1,4-dioxanyl, tetrahydrofuranyl, tetrahydrothiophenyl, tetrahydrothiopyranyl, and the like.
xe2x80x9cHalogenxe2x80x9d and xe2x80x9chaloxe2x80x9d mean fluorine, chlorine, bromine or iodine.
xe2x80x9cHaloalkylxe2x80x9d means an alkyl group, as defmed herein, having one, two, or three halogen atoms attached thereto. Representative haloalkyl groups include chloromethyl, bromoethyl, trifluoromethyl, and the like.
The structurally rigid anionic polymers of this invention may be prepared using methods known in the art for preparing polyamides.
In a preferred aspect of this invention, the structurally rigid polymer is an anionic polymer.
In another preferred aspect, the structurally rigid polymer is a condensation polymer of one or more aryldiamines and one or more cyclic dicarboxylates where at least one of the aryldiamines and cyclic dicarboxylates contains an anionic substitutent.
The structurally rigid nonionic and anionic polymers are preferably prepared by reacting the aryldiamine and cyclic dicarboxylate in the presence of base using an interfacial polymerization technique where the solvent is a mixture of water and an aprotic organic solvent having very little or no miscibility with the water. Examples of suitable organic solvents include methylene chloride, chloroform, carbon tetrachloride, hexane or other aliphatic hydrocarbon solvents, or aromatic solvents such as toluene. A preferred solvent is chloroform. Representative bases include carbonates, bicarbonates, or hydroxides of sodium, lithium, or potassium. Organic tertiary amine bases such as triethylamine, trimethylamine, pyridine, and the like are also suitable. The preferred base is sodium carbonate. Reaction temperatures may range from about 0xc2x0 C. to about 90xc2x0 C. with a temperature of from about 20xc2x0 C. to about 30xc2x0 C. being preferred. Reaction times can very from several minutes to several hours with about two hours being preferred.
In another preferred embodiment, the structurally rigid anionic polymer is a condensation polymer of one or more aryldiamines, one or more cyclic dicarboxylates and one or more cross linking agents, where at least one of the aryldiamines and cyclic dicarboxylates contains an anionic substitutent.
As used herein, xe2x80x9ccross-linking agentxe2x80x9d means a multifunctional compound that when added to the polymerizing aryldiamine and cyclic dicarboxylate results in xe2x80x9ccross-linkedxe2x80x9d polymers in which a branch or branches from one polymer molecule become attached to other polymer molecules. Preferred cross-linking agents include any material containing more than two activated carbonyl groups such as 1,3,5-hexanetricarbonyl chloride, citric acid-tricarbonyl chloride, 1,3,5-benzenetricarbonyl trichloride, and the like or more than two amine groups such as 1,3,5-hexanetriamine. A preferred cross linking agent is 1,3,5-benzenetricarbonyl trichloride.
In another preferred aspect, the aryldiamine is 4,4xe2x80x2-diamino-2,2xe2x80x2-biphenyldisulfonic acid or 4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid.
In another preferred aspect, the cyclic dicarboxylate is benzene-1,4-dicarbonyl chloride.
In another preferred aspect, the structurally rigid anionic polymer is poly(4,4xe2x80x2-diamino-2,2xe2x80x2-biphenyldisulfonic acidbenzene-1,4-dicarbonyl chloride); cross-linked poly(4,4xe2x80x2-diamino-2,2xe2x80x2-biphenyldisulfonic acid/be nzene-1,4-dicarbonyl chloride); poly(4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid/benzene-1,4-dicar bonyl chloride); cross-linked poly(4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid/benzene-1,4-dicarbonyl chloride); poly(4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid/1,2,4,5-benzenetetracarboxylic dianhydride/benzene-1,4-dicarbonyl chloride; or a copolymer of poly(4,4xe2x80x2-diamino-2,2xe2x80x2-biphenyldisulfonic acid/benzene-1,4-dicarbonyl chloride) and poly(4,4xe2x80x2-diaminostilbene-2,2xe2x80x2-disulfonic acid/benzene-1,4-dicarbonyl chloride)
The structurally-rigid anionic polymer of this invention may be used in combination with one or more coagulants and/or flocculants as part of a dual polymer treatment program. The retention and drainage properties of the furnish may also be improved by addition of a microparticle as described herein.
The appropriate dosage of structurally-rigid anionic polymer is determined by adding different doses of the structurally-rigid anionic polymer to a model papermaking slurry either alone, or together with one or more flocculants, coagulants and/or microparticles. The performance of the combined chemical additions is monitored with the focused beam reflectance microscope (FBRM) or other appropriate evaluative measurement (Britt jar, dynamic drainage analyzer, etc.). The range of doses is preferably from about 0.1 about to 50, more preferably from about 0.2 to about 5 and still more preferably about 3 pounds of structurally rigid coagulant/ton product.
xe2x80x9cFlocculantxe2x80x9d means a chemical agent that is added to a papermaking furnish to assist in the agglomeration of small particles and thereby increase the retention and drainage properties of the furnish. The flocculant may be a non-ionic, anionic, cationic or zwitterionic polymer having a molecular weight of at least about 500,000, preferably of at least about 1,000,000 and more preferably of at least about 5,000,000. The flocculant may be used in the solid form, as an aqueous solution, as water-in-oil emulsion, or as dispersion in water.
xe2x80x9cNonionic flocculantxe2x80x9d means homopolymers, copolymers or terpolymers and so on of nonionic monomers. Representative nonionic monomers include acrylamide, methacrylamide, N-tertiary butyl acrylamide, N-vinylformamide, N-vinylpyrrolidone, N-vinylpiperidone, N-vinylcaprolactam, N-vinyl-3-methylpyrrolidone, N-vinypyrrolidone, N-vinylpiperidone, N-vinylcaprolactam, N-vinyl-3-methylpyrrolidone, N-vinyl-5-methylpyrrolidone , N-vinyl-5-phenylpyrrolidone, N-vinyl-2-oxazolidone, N-vinylimidazole, vinylacetate, maleimide, N-vinylmorpholinone, polyethylene oxide (PEO), and the like. Preferred nonionic monomers are acrylamide, methacrylamide and N-vinylformamide. Preferred nonionic flocculants are poly(acrylamide), poly(methacrylamide) and poly(N-vinylformamide).
The dosage of nonionic flocculant is preferably from about 0.001 to about 0.5% (as actives) by weight based on total solids in the slurry, more preferably from about 0.003 to about 0.2% and most preferably from about 0.007 to about 0.1%.
xe2x80x9cCationic flocculantxe2x80x9d means any water-soluble polymer of (meth)acrylamide or any water-soluble polymer of N-vinylformamide or related monomers which carries or is capable of carrying a cationic charge when dissolved in water. Representative cationic copolymers of (meth)acrylamide include copolymers of (meth)acrylamide with dimethylaminoethyl methacrylate (DMAEM), dimethylaminoethyl acrylate (DMAEA), diethylaminoethyl acrylate (DEAEA), diethylaminoethyl methacrylate (DEAEM) or their quaternary ammonium forms made with dimethyl sulfate or methyl chloride, Mannich reaction modified polyacrylamides, diallylcyclohexylamine hydrochloride (DACHA HCI), diallyldimethylammonium chloride (DADMAC), methacrylamidopropyltrimethylammonium chloride (MAPTAC) and allyl amine (ALA).
xe2x80x9cAnionic flocculentxe2x80x9d any polymer comprised of anionic and nonionic monomers means which carries or is capable of carrying a cationic charge when dissolved in water. Representative anionic monomers include acrylic acid, methacrylic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylamidomethylbutanoic acid, maleic acid, fumaric acid, itaconic acid, vinyl sulfonic acid, styrene sulfonic acid, vinyl phosphonic acid, allyl sulfonic acid, allyl phosphonic acid, sulfomethylated acrylamide, phosphonomethylated acrylamide and the water-soluble alkali metal, alkaline earth metal, and ammonium salts thereof. The choice of anionic monomer is based upon several factors including the ability of the monomer to polymerize with the desired comonomer, the use of the produced polymer, and cost. A preferred anionic monomer is acrylic acid. Preferred anionic flocculants are copolymers of acrylamide and acrylic acid.
The dosage of anionic flocculent is from about 0.001 to about 1%, preferably from about 0.01 to about 0.5% and more preferably from about 0.02 to about 0.25% by weight based on total solids in the slurry.
xe2x80x9cZwitterionic flocculentxe2x80x9d means a polymer composed from zwitterionic monomers and, possibly, other non-ionic monomer(s). Representative zwitterionic polymers include homopolymers such as the homopolymer of N,N-dimethyl-N-(2-acryloyloxyethyl)-N-(3-sulfopropyl)ammonium betaine, copolymers such as the copolymer of acrylamide and N,N-dimethyl-N-(2-acryloyloxyethyl)-N-(3-sulfopropyl) ammonium betaine, and terpolymers such as the terpolymer of acrylamide, N-vinyl-2-pyrrolidone, and 1-(3-sulfopropyl)-2-vinylpyridinium betaine. The use of zwitterionic flocculants in papermaking is described in U.S. patent application Ser. No. 09/349,054, incorporated herein by reference.
xe2x80x9cMicroparticlexe2x80x9d means charged materials that improve flocculation when used together with natural and synthetic macromolecules. They constitute a class of retention and drainage chemicals defined primarily by their submicron size. A three dimensional structure, an ionic surface, and a submicron size are the general requirements for effective microparticles.
Microparticle programs enhance the performance of current retention programs and optimize wet end chemistry, paper quality and paper machine efficiency. Microparticles are not designed to be used as a sole treatment. Rather, they are used in combination with other wet end additives to improve retention and drainage on the paper machine. Commonly used microparticles include:
i) copolymers of acrylic acid and acrylamide;
ii) bentonite and other clays;
iii) dispersed silica based materials;
iv) colloidal borosilicate; and
v) naphthalene sulfonate/formaldehyde condensate polymers.
Representative copolymers of acrylic acid and acrylamide are described in U.S. Pat. No. 5,098,520, incorporated herein by reference.
Bentonites useful as the microparticle for this process include: any of the materials commercially referred to as bentonites or as bentonite-type clays, i.e., anionic swelling clays such as sepialite, attapulgite and montmorillonite. In addition, bentonites described in U.S. Pat. No. 4,305,781 are suitable. A preferred bentonite is a hydrated suspension of powdered bentonite in water.
Representative dispersed silicas have an average particle size of from about 1 to about 100 nanometers (nm), preferably from about 2 to about 25 nm, and more preferably from about 2 to about 15 nm. This dispersed silica, may be in the form of colloidal, silicic acid, silica sols, fumed silica, agglomerated silicic acid, silica gels and precipitated silicas, so long as the particle size or ultimate particle size is within the above ranges. Dispersed silica in water with a typical particle size of about 4 nm is available from Nalco Chemical Company, Naperville, Ill.
Representative borosilicates are described in Patent Cooperation Treaty Patent Application No. PCT/US98/19339, incorporated herein by reference. Colloidal borosilicate is available from Nalco Chemical Company, Naperville, Ill.
Naphthalene sulfonate/formaldehyde condensate polymers useful as microparticles are available from Nalco Chemical Company, Naperville, Ill.
The amount of microparticle added is from about 0.05 to about 5.0, preferably from about 1.5 to about 4.5 and more preferably about 2 to about 4.5 pounds microparticle/ton.
xe2x80x9cPounds microparticle/tonxe2x80x9d means pounds of actual microparticle per 2000 pounds of solids present in slurry. The abbreviation for pounds of actual microparticle per 2000 pounds of solids present in slurry is xe2x80x9clbs microparticle/tonxe2x80x9d.
The microparticle is added to the papermaking furnish either before or after the flocculant is added to the furnish. The choice of whether to add the microparticle before or after the flocculant can be made by a person of ordinary skill in the art based on the requirements and specifications of the papermaking furnish.
Optionally, a coagulant is added to the furnish prior to the addition of the structurally-modified water-soluble polymer. Preferred coagulants are water-soluble cationic polymers such as epichlorohydrin-dimethylamine or polydiallyldimethylammonium chloride, alum, polyaluminum chlorides or cationic starch.
Other suitable coagalants include tie structurally rigid cationic polymers desribed in U.S. patent application Ser. No. 09/740,546, filed concurrently herewith, titled xe2x80x9cStructurally Rigid Polymer Coagulants as Retention and Drainage Aids in Papermakingxe2x80x9d, incorporated herein by reference.